The field of the invention is the preparation of methyl tert.-butyl ether (MTB) by the catalytic addition of methanol to isobutene. The state of the art of this preparation may be ascertained by reference to U.S. Pat. Nos. 1,968,601; 2,197,023; 2,282,462; 2,480,940; 2,922,822; 3,121,124; 3,135,807; 3,482,952; 3,718,701 and 3,906,054; the disclosures of which are incorporated herein.
In the catalytic addition of methanol to isobutene, the catalysts employed are acids, such as, for example, H.sub.2 SO.sub.4, as disclosed in U.S. Pat. No. 1,968,601, Lewis acids, such as, for example BF.sub.3 as disclosed in U.S. Pat. No. 2,197,023, platinum metal salts, such as disclosed in U.S. Pat. No. 3,718,701, or also heterogeneous catalysts. Suitable heterogeneous catalysts are, in addition to phosphoric acid on kieselguhr (U.S. Pat. No. 2,282,462), phosphorous-modified zeolites (U.S. Pat. No. 3,906,054), bismuth molybdate and salts of phosphomolybdic acid (U.S. Pat. No. 3,135,807), especially sulfonated organic resins (e.g. U.S. Pat. No. 2,480,940). This group also includes sulfonated polystyrene resins crosslinked with divinylbenzene (U.S. Pat. No. 2,922,822), which can have a gel-type consistency or which can exhibit a sponge structure with macropores to enlarge the surface area and thus to increase the reaction velocity (German Pat. No. 1,224,294, Example 8; U.S. Pat. No. 3,482,952).
In general, the isobutene utilized for the reaction is not pure; rather, isobutene-containing hydrocarbon mixtures are employed from which the isobutene is made to react selectively, inasmuch as the reaction of isobutene with methanol takes place substantially more rapidly than the reaction of the remaining component present in the mixture with methanol.
Accordingly, isobutene can be reacted selectively with methanol not only in a mixture with saturated hydrocarbons, but also in a mixture with unsaturated hydrocarbons, such as propene, butenes (U.S. Pat. No. 3,121,124), or also butadiene (German Published Application DOS No. 2,521,673).
The selectively of the MTB formation is the higher, when using isobutene-containing hydrocarbon mixtures as well as with the use of pure isobutene, the lower the reaction temperature lies. Also the equilibrium of the exothermic ether formation is enhanced by low temperatures. However, with a decreasing temperature, the reaction velocity is reduced so that it becomes increasingly more difficult to attain an approximately complete isobutene conversion--in correspondence with the more favorable equilibrium. Thus, it is hardly possible to operate below the temperature range of 50.degree.-60.degree. C., for example, when using the macroporous, sulfonated organic resins especially effective among the heterogeneous catalysts, if it is desired to attain the thermodynamic equilibrium still within practically feasible contact times. With the equimolar utilization of isobutene and methanol, conversion rates of merely up to at most 92% can be obtained in this connection (DOS No. 2,521,963, page 2).
However, an isobutene conversion which is so inadequate is unsatisfactory with respect to the utilization of the raw material as well as with regard to the quality of the residual hydrocarbons remaining with the use of, for example, a cracked C.sub.4 hydrocarbon cut.
One possibility for completing the isobutene conversion is an increase in the amount of methanol supplied to the reaction. However, with this mode of operation, the MTB thus produced contains considerable quantities of methanol, and extraordinary difficulties are encountered in separating the methanol from the MTB on account of the formation of an azeotrope with a high MTB content. Thus, in order to separate methanol from MTB, for example, suggestions have been advanced to conduct an extractive distillation with dimethyl sulfoxide or also a water washing process (DOS No. 2,246,004; Japanese Laid-Open Application No. 73-00509).